Efficient intersystem crossing and tunable ultralong organic room-temperature phosphorescence via doping polyvinylpyrrolidone with polyaromatic hydrocarbons

Polymer-based pure organic room-temperature phosphorescent materials have tremendous advantages in applications owing to their low cost, vast resources, and easy processability. However, designing polymer-based room-temperature phosphorescent materials with large Stokes shifts as key requirements in biocompatibility and environmental-friendly performance is still challenging. By generating charge transfer states as the gangplank from singlet excited states to triplet states in doped organic molecules, we find a host molecule (pyrrolidone) that affords charge transfer with doped guest molecules, and excellent polymer-based organic room-temperature phosphorescent materials can be easily fabricated when polymerizing the host molecule. By adding polyaromatic hydrocarbon molecules as electron-donor in polyvinylpyrrolidone, efficient intersystem crossing and tunable phosphorescent from green to near-infrared can be achieved, with maximum phosphorescence wavelength and lifetime up to 757 nm and 3850 ms, respectively. These doped polyvinylpyrrolidone materials have good photoactivation properties, recyclability, advanced data encryption, and anti-counterfeiting. This reported design strategy paves the way for the design of polyvinylpyrrolidone-based room-temperature phosphorescent materials.

promising promising red RTP emifters with excepfional performance."These values seem "strange".4) Methods paragraph: "Unless otherwise stated, all materials are purchased from commercial sources without further eradicafion.";"Grinding powder is used for all tests, and the amounts of samples for PL measurement are similar.Time resolved PL decay kinefics and PL spectra were recorded in different temperatures by the Edinburgh company."5) Supporfing: Figure S4: annealed at which temperature?In the manuscript the annealing is performed to erase the label…; Fig S5 right "absorance"; "Table.S1.Energies of S1, T1, T2, Energy Gaps (in eV) between S1 and T1/T2 States and SOC (Inter-system crossing rate constant) of …" these are spin orbit coupling costants….;SOC values of VP in Table 1 are "strange"; Fig. S13b: what is the difference with Fig. S4 in the annealing procedure????; Fig. S8: Py/PMMA or Py/KBr???? 6) References 59 and 60 are lacking

Reviewer #3 (Remarks to the Author):
This manuscript by Li Dang et al. provided a polymer-based RTP system using PAHs as the dopants and the PVA as the matrix.Though doping system using PAHs such as Py and Co have been wildly reported, this work focuses on the effect of PVP polymer on dopants.The authors provided an improved mechanism using 1CT/3CT as a step between Sn and T1 of the guest.They aftempted to prove through transient absorpfion and theorefical calculafions that the 1-vinyl-2-pyrrolidinone (VP) unit and gust formed the CT states.However, the evidence provided by the authors is insufficient, and I believe that non aromafic VP cannot form a CT state with a PAH.After abandoning the mechanism of polymer matrix providing CT effect, this work only introduced a polymer-based RTP system, which obviously lacks sufficient novelty.Therefore, I would like reject this manuscript.
1) The TA spectra only show guest radical cafion.I think this evidence is not enough for the host-guest CT states, because CT absorpfion and power-law decay were not observed.2) CT states consists one host and one guest molecule.But the theorefical calculafion is based on a set of several VP molecule.I think it doesn't make sense.For one VP molecule, it is impossible to form a CT state with a PAH.
3) The authors says "…SOC values are quite small … that ISC is difficult to occur…".Actually, most of these guests can show RTP after doped into polymers such as PVA without the so-called CT states.4) Even if the CT states exists, the proposed mechanism (figure 4d) is incorrectly expressed.It is "energy transfer" from 1CT/3CT to T1, not "charge recombinafion".And 1CT/3CT is generated through a serious steps containing charge separafion, charge transfer and charge recombinafion.5) Based on the doping principle of different guests and the decided host molecule (pyrrolidone), how to verify the best intermolecular distance and interacfions between host and guest?Because the author only chose a mass rafio of 1% to prepare polymer films with host molecule of the same molecular weight.

Reviewer #4 (Remarks to the Author):
I co-reviewed this manuscript with one of the reviewers who provided the listed reports.This is part of the Nature Communicafions inifiafive to facilitate training in peer review and to provide appropriate recognifion for Early Career Researchers who co-review manuscripts.

▣ Detailed Responses to the Reviewer #1's Comments:
Comment 1: Page 2 line 46, the first letter of Benzophenone should be lowercase.The expression of the entire text needs further verification.

Response:
We are sorry for this mistake, we have corrected the description (page 2 in the revised text) as "These materials can be easily fabricated by mixing pyrene (electron donor) and benzophenone (electron acceptor) using the melt-casting method."In addition to this, we corrected all typos and polished the whole manuscript based on the reviewers' suggestions.We highlighted the corrections in the revised manuscript and thank the reviewers for this suggestion.
Comment 2: Some typical literatures of polymer-based RTP materials with color tunable afterglow and near-infrared emission should be cited in the article to be more convincing, such as Adv.Mater. 2022, 34, 2108333;Chem. Eng. J. 2023, 469, 143931;Eur. Polym. J. 2024, 202, 112600.Response: Many thanks to the reviewer for your suggestion, which we feel is very reasonable.Therefore, after carefully reading the relevant literature suggested by the reviewer, we have cited them in the appropriate places.(page 3 in the revised text) as "More to the point, only sporadic examples have emitted ultra-long wavelength afterglows so far, 56-58 and most of them are organic crystals.Comment 3: Fig. 2b does not show a clear trend in afterglow variation.The afterglow videos of the doped PVP films should be added in the supporting information.

Response:
We thank the reviewer for this reminder.We have added the afterglow videos of the doped PVP films in the supporting information as suggested, as shown in Supplementary Video.
Comment 4: The quantum yield of the doped PVP films should be measured.

Response:
We sincerely thank the review for the valuable suggestions, and the photophysical data Table 1 for doped PVP films has been added to the manuscript as suggested.Response: Thanks to the reviewer for your suggestion.We have changed the title of the manuscript to"Efficient intersystem crossing and tunable ultralong organic room temperature phosphorescence via doping polyvinylpyrrolidone with polyaromatic hydrocarbons".
Comment 2: The introduction paragraph needs to be better written and organized: some sentences need to be anticipated, others postponed.Moreover: "…due to special luminescent time."The adjective special is meaningless…; "According to the Jablonski diagram, there are two strategies to achieve good phosphorescence: one is to promote the intersystem crossing (ISC) rate from excited singlet state to excited triplet state; the other is to enhance the stabilities of triplet exciton of organic molecules."strategies should be changed into "requirements", moreover these requirements hold for every phosphorescent material; "One is that lower T1 implies a larger band gap (ΔEst) between S1 and T1, which is not favorable to intersystem crossing (ISC) of excitons, not to say phosphorescence.The other is that lower T1 is more likely to lead to non-radiative depletion of excitons, which results in a significant reduction in the lifetime and intensity of the phosphorescence."What does "not to say phosphorescence" mean?
"This study gives us the hint that to make good use of efficient fluorescent molecules such as aromatic hydrocarbons as the guests, which are highly conjugated with very low T1 energy levels which is essential for long-wavelength phosphorescence."Please re-write; why Sn has been added in the left part of Fig. 1a? this is misleading.

Response:
We sincerely thank the reviewer for careful reading.
(3) Since phosphorescence originates from the radiative transition of excited triplet excitons.But when the band gap (∆Est) between S1 and T1 is too large, it is difficult for ISC to occur, and it is difficult to produce triplet excitons.This makes it difficult to observe phosphorescence.Therefore, we have revised "not to say phosphorescence" to "making it difficult to observe phosphorescence."(page 2).
(4) We have revised "This study gives us the hint that to make good use of efficient fluorescent molecules such as aromatic hydrocarbons as the guests, which are highly conjugated with very low T1 energy levels which is essential for long-wavelength phosphorescence." to "This study gives us the hint that to make good use of polyaromatic hydrocarbons as the guests with very low T1 energy levels due to the highly conjugated nature of these molecules, which is crucial for long-wavelength phosphorescence."(page 2).
(5) Thanks to the reviewer for this reminder.We have corrected the Fig. 1a as follows.
Comment 3: Results and discussion paragraph: "First of all, when different fluorescent PAHs molecules as guests (Fig. 2a) and PVP (Mw = 40,000-45,000 g/mol) as host are mixed at a mass ratio of 1% to prepare polymer films, and fluorescence emission under 365 nm UV excitation is observed (Fig. 2b UV on and Supplementary Fig. 1)."polymer films should be better substituted by "blended films" or "doped films"; "Moreover, the luminescent lifetimes gradually increase with longer irradiation time (Fig. 2d).In the meantime, good to excellent phosphorescence quantum yields (Φp) of these activated films are determined and listed in Table 1."What is the irradiation time at which values listed in table 1 are given; sentence "BePh/PVP films and found that the RTP was further enhanced under vacuum conditions (Supplementary Fig. 4).The weak phosphorescence of the pristine samples may be caused by the energy transfer from the triplet excitons of the guest compound to the O2 molecule in the PVP.These results conversely indicate that the luminescence involves triplet stats and explain the results in Fig. 2c and 2d that the luminescent intensities and lifetimes are enhanced along with the increase of irradiation time allowing O2 consumption."Is not clear at all: "The weak phosphorescence of the pristine samples may be caused by the energy transfer from the triplet…" means "The quenching of the phosphorescence of the pristine samples may be caused by the energy transfer from the triplet.."? the use of "conversely" is not clear; "1-vinyl-2-pyrrolidinone (VP) as a monomer for PVP has higher highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels than PVP."; regarding Fig. 2c (Delayed emission spectra of these doped PVP films after photoactivation under UV irradiation at 365 nm at 105 different times, delay time: 1 ms) due to impurity/reproducibility concerns it is reasonable to wonder if spectra have been collected for different samples; "The energy differences between the HOMOs of the guests and LUMO of PVP could be small enough for intermolecular charge transfer (Supplementary Fig. 11)."Which is a reasonable limit?A reference should be added; "To validate the above experimental data, the excited state calculations have been performed on the guests and the complexes."Wouldn't it be better to use the word "pair" or "dimer"?"Therefore, the charge transfer state opens a new trajectory for the ISC of the guests, which will dramatically improve the ISC process of PAHs guest molecules.More importantly, the calculated triplet state energies essentially match the phosphorescence colors, which also suggests that the phosphorescence emission originates from the guest triplet state."We would change suggests with confirms or supports since experiments in solution at 77 K have already supported this mechanism; "After 2 minutes of irradiation, the UV light is removed, and the drawn patterns relevant afterglows respectively (Fig. 5a and Supplementary Fig. 12).";"By mask illumination of the samples, any pattern can be applied as phosphorescent information storage.Py/PVP and FlAn/PVP films are covered with mask A and then irradiated by the 365 nm UV light (9 mWcm -2 ) for 120 seconds.
Remove the mask after the writing is completed, and print a Chinese character '鑫' on the film.";"Afterward, the pattern is erased via annealing at 80 for 5 min, and a renewed film is obtained by replacing mask A with mask B and repeating the above procedures, the quick response code can be clearly printed."The annealing at 80°C for 5 min to erase the pattern is mentioned "suddenly" and for the first time at this point of the manuscript: how has this temperature been selected?Have thermal analysis of the films been performed?"Among them, the RTP emissions of Py/PVP, BeAn/PVP, DBeCh/PVP and BePe/PVP materials are all in the red region with RTP lifetimes 198 of 358, 139, 468 and 214 ms, and afterglow durations of 4, 3, 7 and 10 s, respectively, which are very promising red RTP emitters with exceptional performance."These values seem "strange".
Response: Thanks to the reviewer's suggestions.
For the CT between host and guest in Py/PVP doped films, the population and transformation of the CT absorption is usually very fast.As shown in Fig. S7, the transient absorption peaks gradually rise up and have a red-shift from 433 to 441 nm for Py/PVP film and from 521 to 525 nm for BePe/PVP film at the beginning delay times.These steps are attributed to the CT process in the doped film.
In doped systems, where the host is the vast majority and the guest content is very small, it is usually very difficult to observe the cationic or anionic radicals by the transient absorption when it undergoes intermolecular charge transfer in the excited state.But in our system, the conjugation of the guest molecule is very good and aromatic, so we can observe the cationic radical of the guest.However, the host molecule is not conjugated, so the molar extinction coefficient of its anionic radicals will be relatively small, it is difficult to detect the corresponding anionic cations.Nevertheless, the detection of the cationic radical signal of the guest molecule in the transient absorption spectra can further prove that the intermolecular charge transfer process in the excited state has occurred between host and guest in the doped systems.For the CT between host and guest in Py/PVP doped films, the population and transformation of the CT absorption is usually very fast.As shown in Supplementary Fig. 7, the transient absorption peaks gradually rise up and have a red-shift from 433 to 441 nm for Py/PVP film and from 521 to 525 nm for BePe/PVP film at the beginning delay times.These steps are attributed to the CT process in the doped film.Generally speaking, the host is the vast majority and the guest content is very small in the doped systems.Therefore, it is usually very difficult to observe the cationic or anionic radicals by the transient absorption when it undergoes intermolecular charge transfer in the excited state.But in our system, the conjugation of guest molecules is very good and aromatic, thus the cationic radical of the guest is directly observed by fs-TA.However, the host molecule is not conjugated, thus the molar extinction coefficient of its anionic radicals will be relatively small, it is difficult to detect the corresponding anionic cations.

Fig. S7 .
Fig. S7.Shown are fs-TA spectra of Py/PVP (a) and BePe/PVP (b) films at the beginning delay times after the activation with 365 nm UV light, respectively.

Table 1 .
Photophysical data of the polymer based ORTP materials.

of Fluo.: 375 nm; Ex. of Phos.: 365 nm; Delayed time: 1 ms. ▣ Detailed Responses to the Reviewer #2's Comments: Comment
1: We would suggest to modify the title into: "Efficient intersystem crossing and tunable ultralong organic room temperature phosphorescence via doping polyvinylpyrrolidone with polyaromatic hydrocarbons"